In a case that a potential higher than a basic potential of unit battery (cell) is necessary, such as a hybrid vehicle using a lithium ion cell as a power source, it is common to use a plurality of unit batteries which is connected in series.
However, even though the batteries are produced with the same structure via a typical production method using the same anode, cathode and electrolyte material, a difference in charging or discharging (and self discharging) characteristics exists between each of the batteries connected in series.
Therefore, a potential difference can exist between the unit batteries when using the batteries connected in series.
Even if one battery of the unit batteries connected in series is perfectly discharged regardless of a potential of other battery, a total voltage (total voltage of batteries connected in series) becomes zero so that the battery is need to be recharged. Upon recharging the battery, since the potentials of the batteries are different from one another, there is a problem of over-charging in that the battery may reach a prescribed voltage in advance, as well as a problem of charge inefficiency in that some batteries may not reach the prescribed voltage even in an existence of an over-charging.
Further, if the number of charging/discharging times becomes large, degradation is caused in materials composing the battery and thus a property of the battery is varied, and as a result, such degradation situation is responsible for further increasing a difference in individual cells.
In order to address such problems, there are proposed various charge-equalization apparatuses which can accomplish charge equalization of the batteries connected in series.
As an example, Korean patent Laid-Open No. 2003-0096978 is directed to a system consisted of a plurality of unit cells, a charging means, a discharging means, series-parallel converting switch, wherein the charging is performed by uniformly discharging each of the plurality of unit cells and then connecting the discharged unit cells in series using the series-parallel converting switch. Korean Patent Laid-Open 2007-0064244 is directed to a system which comprises a cell unit, a field effect transistor unit connected to the cell unit, an amplifying unit connected to the field effect transistor unit, a multiplexer unit controlling an output signal of the amplifying unit, a comparing unit comparing a difference between voltage signals of the cell unit, an A/D converter converting the output of the comparer to a digital signal, a Micom unit inputting the signal outputted by the A/D converter and outputting signal corresponding to charging/discharging condition, a switching unit operated according to a signal of the Micom unit to provide battery equalization current, and a known charging/discharging circuit.
Further, Japanese Patent Laid-Open No. 1998-032936 is related to a system consisted of a plurality of unit cells, a means for detecting remaining capacity of each unit cell, charge and discharge replacing means for performing charging and discharging of each unit cell, a controller controlling charging and discharging of each unit cell, and direct current/direct current converter performing charging and discharging of each unit cell independently. Japanese Patent Laid-Open No. 2004-194410 is related to a system consisted of at least two unit cell groups, a means for detecting a difference between currents flowing each of a first cell group, and a second cell group, a means for controlling charging/discharging current of the cell group based on the current difference.
However, in the prior charge equalizing apparatuses, since each of batteries connected in series is equipped with a charge equalizing apparatus to perform charging or discharging of individual battery, there are problems in that complexity and volume of the charge equalizing apparatus are increased thereby decreasing productivity and increasing production costs, and elements composing the charge equalizing apparatus or switch module must withstand a higher voltage stress.
The two-stage charge equalization apparatus and method according to the present invention is characterized in that total batteries are classified into battery modules and each of the battery modules shares the DC/DC converter thereby reducing complexity of the charge equalization apparatus and also obtaining efficient charge equalization, the two-stage DC/DC converter is structured such that single DC/DC converter is connected to the input of the DC/DC converter shared for each module thereby reducing the voltage stress of the DC/DC converter and control switch which typically have high voltage stress, and the DC/DC converter shared for each module is connected in parallel to individual battery thereby improving charging efficiency.